ABSORPTION SPECTRA OF SULPHUROUS ACID AND SULPHITES. 2907CCLXXI1.-The AbsoqAon SpectraAcid and Suiphites.By ROBERT WRIGHT.of SulphwousIN a fcrmer paper (this vol., p. 669) dealing with the relationbetween the absorption spectra of various acids and their alkalisalts, the absorption curves of sulphurous acid and sodium sulphitewere shown. I n view of the doubtful constitution of these sub-stances, and as their light absorptive powers are so fundamentallydifferent, the acid showing selective, and the salt only generalabsorption ; i t was considered of importance t o investigate theirspecial case more closely.Potassium sodium sulphite, prepared either from sodium hydro-gen sulphitel and potassium hydroxide o r from potassium hydrogensulphite and sodium hydroxide, gave a spectrum identical with thatof sodium sulphite.Most sulphites of the bivalent metals are too insoluble to admitof investigation ; magnesinm sulphite is, however, sparinglysoluble, and on photographing its M/lOO-solution it was found t ogive the same spectrum as the alkali sulphites.It seemed possible that sulphurous acid might have a structuresimilar to that of the sulphonic acids, for it will be seen on refer-ence to the paper already mentioned that benzenesulphonic acidand its salt both give a band in the same region of the spectrum,but at a greater dilution than sulphurous acid.On examination,however, the sodium salt of ethylsulphonic acid was found to bequite diactinic, so that the occurrence of the band in the aromaticcompound must be attributed t o the phenyl nucleus, which, as iswell known, almost invariably causes selective absorption.Ethyl sulphite, from thionyl chloride, was also examined inalcoholic solution, and was found t o be practically diactinic atM / 10-dilutionStewart and Macbeth, in an unpublished research, observed thatgaseous sulphur dioxide gave a band in the ultra-violet. It wastherefore thought advisable to compare this band with that ofaqueous sulphurous acid.The investigation was carried out by means of the followingapparatus. A tube of about 2 cm.in diameter and exactly 10 cm.long has its ends closed with quartz windows. This observationcell has two side-tubes, one connected with a source of the gas andthe other with a manometer and pump.The apparatus is fittedwith three taps, one between the source of the gas and the cell2908 WRIGHT: THE ARSORPTION SPECTRA OFone between the cell and the manometer, and the third betweenthe manometer and pump. .All connexions in the apparatus wereof glass.i n using the apparatus, all the taps are opened and the air isswept out by a stream of sulphur dioxide; the taps are now closed,the apparatus is connected with the pump, and a series of photo-graphs taken through the cell a t different pressures, which areobserved by means of the manometer. Taking 2.9266 grams as theweight of a litre of sulphur dioxide a t N.T.P., the normality of thegas under these standard conditJons is 2*9266/ 64 = 0.04572N. Athickness of 100 imn.of a gas the normality of which is 044572corresponds with 4.57 mm. of N-concentration. By a similar cnlcu-Frequeiicies.30 32 34 36 38 40 42 44Full curve = Sulphur dioxide iir, aqueous solution.Dotted ,, = Gaseous Sulphw dioxide.Dash ,, = Alkali sulphites.lation the 10 cm. thicknesses a t different pressures can be reducedto the corresponding thicknesses of N-gas. The curve drawn fromthese results is shown in comparison with that of aqueous sulphur-ous acid, and it will be seen that they strongly resemble oneanother : for although differing slightly in persistence, they corre-spond in their positions in the spectrum and in the thicknesses a twhich they occur.A few comparison photographs were also made, using air as adiluent. For this purpose the tube was filled with gas as before,the pressure reduced, and a photograph taken; air was nowadmitted t o the cell by raising a tap out of its seat, and a secondexposure made.Since the gas in the cell is under less than atmo-spheric pressure, it cannot escape during this process, and so thSULPRUROUS ACID AND SULPHITES. 2909second exposure is made through the same amount of sulphurdioxide as the first; only, in the latter case it3 is diluted with airinstead of being under diminished pressure. I n the half-dozencomparisons made no difference could be detected between the twosets of photographs.The result seems t o indicate that aqueous sulphurous acid consistslargely of uncombined sulphur dioxide molecules. The solution wasalso examined a t zero, by surrounding the cell with ice, in orderto see if the solution of the crystalline compound which separatesat that temperature gave the same spectrum as the solutlion underordinary conditions. No difference in the two spectra could bedetected.From this work the conclusion may be drawn that an aqueoussolution of sulphur dioxide consists to a large extent of uncombinedgas molecules.Q U I ~ N ’ S UNIVERSITY,B 14: LFASTPRINTED IN GREAT BRITAIN BYRICHARD CLAY AND SONS, I,IMITED,HRUNSWICK STREET, STAMFORD S'CKEET, S.E.,AND BUNGAY SUFFO1.